Flexible materials, especially packaging materials of paper, plastic films or metal foils, or materials with an irregular surface such as, e.g., corrugated board, are in practice printed primarily by the flexographic printing process. Printing plates that have high elasticity and flexibility are needed for this process. The rubber plates used almost exclusively for this purpose previously have recently been displaced by photopolymer printing plates, which can be produced at much lower cost. Photopolymer printing plates for flexographic printing consist of a light-sensitive layer usually applied onto a layer support. Essential constituents of the photosensitive layer are a light-hardenable compound, an elastomeric binder, and a photoinitiator or initiator system. Photopolymer printing plates whose binders are block copolymers whose polymer blocks contain polystyrene and polyisoprene and/or polybutadiene, have proven particularly good in practice. The arrangement, composition, and proportions of the individual polymer blocks can vary within wide limits. Thus, e.g., homopolymer blocks of styrene and homopolymer blocks of isoprene and/or butadiene can be present. The block copolymer can also be made up, however, of copolymer blocks of styrene and isoprene and/or butadiene and homopolymer blocks of butadiene and/or isoprene. Typical examples of such elastomeric binders are described, e.g., in German Pat. No. 2 215 090 (U.S. Pat. No. 4,323,636), German AS No. 2 815 678 (U.S. Pat. No. 4,197,130) and European Pat. No. 00 75 236 (U.S. Pat. No. 4,430,417).
To produce the printing plate, the photopolymerizable element is exposed through an original and the unexposed areas are washed off with a suitable solvent. To produce an optimum printing surface the plate is then chemically after-treated and subsequently post-exposed overall to actinic radiation. These treatment steps can optionally also be carried out with the plate mounted on the printing cylinder. For printing, the flexographic printing plate or, if the printing plate is produced on the printing cylinder, the photopolymerizable printing element, is mounted on the printing cylinder or on an endless belt. In practice, printing plates are frequently composed of individual parts. The necessity of placing them together may be, e.g., due to the motif to be printed itself, or for reasons of cost or to achieve larger plate formats. As is known, printing plates are not available in all desired sizes, since the format size is limited above all by the size of the processing and exposing apparatus available. Especially when packaging material for large objects must be printed, there is a need for formats that are not easily obtainable. For a number of applications it is also necessary to cover the printing cylinder with the photopolymerizable printing element or printing plate endlessly. For this, the printing plate or the photopolymerizable element is laid around the printing roll and the ends are pushed together. They are then fixed by taping them together, using double-sided adhesive tapes. If printing plates consisting of individual parts are used, here too the edges of the individual parts abut at the respective connection points and are fixed on the printing cylinder with double-sided adhesive tapes. As a result of their elasticity and restoring force, however, the flexographic printing plates tend to become detached from the printing cylinder during the printing process, especially at the neighboring ends of the abutting edges, which then stand up. Because of this, printing ink can get between the printing plate and the adhesive tape and the solvents contained in the printing inks impair the adhesive power of the double-sided adhesive tape, which can finally lead to the printing plate becoming detached completely.
A process to connect the edges of photopolymerizable printing elements seamlessly by means of a combined pressure- and heat-treatment is known from German Pat. No. 2 722 896. This process requires a great deal of apparatus, however, usually requires a subsequent smoothing process, as well as further post-treatment steps, and is therefore only used in special cases that require a seamless joining of the abutting edges. The edges of printing plates cannot be joined by this process.
In order to prevent detachment, attempts have also been made to seal the gap between the abutting edges by inserting filling material, e.g., adhesive cement, or filling compositions based on rubber, polyacrylate, or cyanacrylate, epoxy resins, and the like. In this manner, however, non-printing connection points are obtained whose strength and elastic properties are not adequate to withstand the forces acting on the printing plate during printing and/or the repeated manual mounting and unmounting, so that the connection points break open again after only a short time.
As is known, flexographic printing plates must fulfill a large number of requirements, caused especially by the materials to be printed. On the one hand they must be soft enough to fit the irregular surface of the materials to be printed, but on the other hand they must be resistant to abrasion, i.e., must have a certain strength. A high tensile strength is also required, combined with rapid ability to recover, as well as resistance to the solvents of the printing ink.
In-house tests have shown that a printing edge joint suitable for practical use can only be achieved if the connection point has essentially the same elastic properties, especially tensile strength and restoring force, the same strength, and the same solvent behavior, as the printing plate itself.
It was therefore the object of the present invention to give a process suitable for joining the edges of photopolymerizable printing elements and of printing plates, especially of printing areas of the printing plates, that contain as elastomeric binder a thermoplastic, elastomeric block copolymer having polymer blocks of polystyrene and polyisoprene and/or polybutadiene or their copolymers, and that leads to a printing edge joint whose connection point corresponds to the printing plate in its elastic properties, its strength, and its solvent behavior.